If a person had already recited Birkat Ha'levana, and his friend cannot, for whatever reason, recite the Beracha, may the first person repeat the Beracha for his friend, who will listen, answer "Amen" and thereby fulfill his obligation?
A general Halachic principle establishes that "Keivan She'yasa Mosi" – even if somebody had already fulfilled a given Mitzva, he may repeat the Mitzva and its accompanying Beracha to allow somebody else to fulfill the Mitzva. Thus, for example, a person who had already recited Kiddush may repeat it for others who have yet to recite or hear Kiddush. Similarly, one who had already heard or read Megilat Ester on Purim may read it again with the Berachot for those who had not fulfilled their obligation. This applies as well to Shofar: one who had blown the Shofar early in the day of Rosh Hashanah may blow it again and recite the Berachot to fulfill the obligation on behalf of those who have yet to hear the Shofar. Seemingly, then, a person who had recited Birkat Ha'levana should be permitted to repeat the Beracha on behalf of somebody else who has yet to fulfill this obligation.
Hacham Ovadia Yosef, in his work Yabia Omer (vol. 9, Orah Haim section, 3:2), relates that this question was posed to him by his mentor, Hacham Ezra Atiya. Hacham Ovadia replied that this issue hinges on a dispute among the Rishonim as to whether the aforementioned principle of "Keivan She'yasa Mosi" applies to "Birkot Ha'shevah" – Berachot recited to give praise for a given experience or phenomenon. Rabbi Menahem Meiri (France, 1249-1315) held that when it comes to these Berachot, one cannot repeat the Beracha for the purpose of fulfilling his fellow's obligation. Hence, a person who had recited Birkat Ha'levana, in which we give praise to God for the phenomenon of the new moon, cannot repeat the Beracha to fulfill his fellow's obligation. The Meiri's ruling would also affect the case of a person who cannot, for whatever reason, recite Asher Yasar after using the restroom, and wishes to have somebody else – who has not just used the restroom – recite the Beracha on his behalf. Asher Yasar, too, falls under the category of "Birkot Ha'shevah," as it gives praise to the Almighty for enabling the human body to eliminate waste. Hence, according to the Meiri, one who has not just used the restroom cannot recite the Beracha on behalf of somebody who has.
The Ritva (Rabbi Yom Tov Ashbili, Spain, 1250-1330) disagreed, and held that the rule of "Keivan She'yasa Mosi" applies to all Berachot, including "Birkot Ha'shevah." In his view, then, one may repeat Birkat Ha'levana or Asher Yasar for the purpose of fulfilling his fellow's obligation.
As for the final Halacha, Hacham Ovadia applies here the famous principle of "Safek Berachot Le'hakel," that we must not recite a Beracha if there is some question concerning its validity. Since the Meiri would not allow repeating Birkat Ha'levana or Asher Yasar in such a case, Hacham Ovadia rules that one should not repeat these Berachot on behalf of somebody else. This ruling is cited as well by Hacham Ovadia's son, Hacham Yishak Yosef, in his work Kol Torah (p. 45). And although Rabbi Moshe Halevi, in his work Birkat Hashem (vol. 4, p. 67), disputes this position and allows repeating Birkat Ha'levana or Asher Yasar in such a case, one should follow Hacham Ovadia Yosef's ruling and not repeat these Berachot to fulfill somebody else's obligation.
Summary: Somebody who recited Birkat Ha'levana should not repeat the Beracha for another person who has yet to fulfill his obligation. Similarly, one who is not currently required to recite Asher Yasar may not recite the Beracha on behalf of somebody who has just used the restroom and is thus obligated to recite the Beracha.
